The present invention relates to ionized deposition processes, and more particularly, to a method and apparatus for sputtering a coil in the fabrication of semiconductor devices.
To improve bottom coverage of high aspect ratio vias, channels and other openings in a wafer or other substrate, the deposition material may be ionized in a plasma prior to being deposited onto the substrate. The ionized deposition material may be redirected by electric fields to ensure more material reaches the bottom areas. It has been found that it is desirable to increase the density of the plasma to increase the ionization rate of the sputtered material in order to decrease the formation of unwanted cavities in the deposition layer. As used herein, the term xe2x80x9cdense plasmaxe2x80x9d is intended to refer to one that has a high electron and ion density.
There are several known techniques for exciting a plasma with RF fields including capacitive coupling, inductive coupling and wave heating. In a standard inductively coupled plasma (ICP) generator, RF current passing through an antenna in the form of a coil surrounding the plasma induces electromagnetic currents in the plasma. These currents heat the conducting plasma by ohmic heating, so that it is sustained in steady state. As shown in U.S. Pat. No. 4,362,632, for example, current through a coil is supplied by an RF generator coupled to the coil through an impedance matching network, such that the coil acts as the first windings of a transformer. The plasma acts as a single turn second winding of a transformer.
A high density plasma typically requires the chamber to be operated at a relatively high pressure. As a result, the frequency of collisions between the plasma ions and the deposition material atoms is increased and the scattering of the deposition atoms is likewise increased. This scattering of the deposition atoms typically causes the thickness of the deposition layer on the substrate to be thicker on that portion of the substrate aligned with the center of the target and thinner in the outlying regions. Such nonuniformity of deposition is often undesirable in the fabrication of semiconductor devices. Thus, although ionizing the deposition material in a high density plasma can facilitate deposition of material into high aspect ratio channels and vias, many sputtered contact metals have a tendency to deposit more thickly in the center of the wafer as compared to the edges.
The deposition layer can be made more uniform by reducing the distance between the target and the substrate, which reduces the effect of plasma scattering. However, decreasing the distance between the target and the substrate decreases the ionization of the deposition material and, hence, increases the formation of unwanted cavities in the deposition layer. For this reason, it is important to maintain a relatively high plasma ionization rate to minimize cavity formation in the deposition layer.
As described in copending application Ser. No. 08/680,335, entitled xe2x80x9cCoils for Generating a Plasma and for Sputtering,xe2x80x9d filed Jul. 10, 1996 and assigned to the assignee of the present application, which application is incorporated herein by reference in its entirety, it has been recognized that the coil itself may provide a source of sputtered material to supplement the deposition material sputtered from the primary target of the chamber. Application of an RF signal to the coil can cause the coil to develop a negative bias which will attract positive ions which can impact the coil causing material to be sputtered from the coil. Because the material sputtered from the coil tends to deposit more thickly at the periphery of the wafer, the center thick tendency for material sputtered from the primary target can be compensated by the edge thick tendency for material sputtered from the coil. By shaping the coil in the form of a ribbon, it has been found that uniformity can be improved.
However, because relatively large currents are passed through the coil to energize the plasma, the coil often undergoes significant resistive heating. In addition, ions impacting the coil can further heat the coil if the coil is used as a sputtering source. As a result, the coil can reach relatively high temperatures which can have an adverse effect on the wafer, the wafer deposition process or even the coil itself. Moreover, the coil will cool once the deposition is completed and the current to the coil is removed. Each heating and subsequent cooling of the coil causes the coil to expand and then contract. This thermal cycling of the coil can cause target material deposited onto the coil to generate particulate matter which can fall onto and contaminate the wafer.
To reduce coil heating, it has been proposed in some applications to form the coil from hollow tubing through which a coolant such as water is passed. However, such tubing typically has a relatively poor sputtering rate and therefore is not well suited to sputtering. As a consequence, improvements in uniformity may be more difficult to achieve, particularly if a relatively high rate of coil sputtering is needed.
It is an object of the present invention to provide an improved method and apparatus for sputtering which obviates, for practical purposes, the above-mentioned limitations, particularly in a manner requiring a relatively uncomplicated arrangement.
These and other objects and advantages are achieved by, in accordance with one aspect of the invention, an RF coil disposed within a vacuum chamber, between a target and substrate support, wherein the coil has both a coolant carrying channel and a sputtering surface shaped to enhance sputtering of the coil. The coil sputtering surface provides for sputtering at sufficiently high rates to reduce non-uniformity in the deposition layer on the substrate. At the same time, a coolant carrying channel defined within the coil facilitates heat transfer from the sputtering surface to the coolant to prevent the coil from reaching undesirably high temperatures during the sputtering process and prevent thermal cycling.
In one embodiment, the coil comprises two coils joined together to form a hybrid coil. A first RF coil, which sputters deposition material onto a substrate, is disposed within the vacuum chamber and extends through a space defined between the target and substrate. This first coil may be a ribbon-shaped coil which is especially suited for providing edge-thick sputter depositing of material at a rate sufficient to offset the center-thick deposition from the target, thereby enhancing the uniformity of the deposition layer. A second tubular-shaped coil is thermally and electrically coupled to the first coil to absorb heat from the first coil. Such an arrangement facilitates economical manufacture of the hybrid coil as discussed below.